Filter by templated schema name for postgres queries (#21635)

There were certain queries which did not filter
by the current schema, but hardcoded public schema.

Apart from that, we always filter with the current schema
which allows connection proxies to run multiple MM within
a single DB.
```release-note
NONE
```

db/migrations/postgres/000033_create_sidebar_channels.up.sql

@@ -14,6 +14,7 @@ BEGIN
     SELECT count(*) != 0 INTO col_exist_and_type_different
     FROM information_schema.columns
     WHERE table_name = 'sidebarchannels'
+    AND table_schema = current_schema()
     AND column_name = 'categoryid'
     AND data_type = 'character varying'
     AND NOT character_maximum_length = 128;